1. Field of the Invention
The present invention relates to a novel steam turbine, and more specifically, to a steam turbine provided with a rotor integrating high and low pressure portions fabricated from Ni-Cr-Mo-V low alloy steel having superior high temperature strength and toughness, the rotor shaft thereof, heat resisting steel, and a manufacturing method thereof.
2. Description of the Prior Art
In general, Cr-Mo-V steel specified in accordance with ASTM (Designation: A470-84, Class 8) is used as a material of a high pressure rotor exposed to high temperature steam (steam temperature: about 538.degree. C.) and 3.5 Ni-Cr-Mo-V steel specified in accordance with ASTM (Designation: A470-84, Class 7) is used as a material of a low pressure (steam temperature: about 100.degree. C.) rotor. The former Cr-Mo-V steel is superior in high temperature strength, but inferior in low temperature toughness. The latter 3.5 Ni-Cr-Mo-V steel is superior in low temperature toughness, but inferior in high temperature strength.
A turbine having a large capacity comprises a high pressure portion, an intermediate pressure portion, and a low pressure portion in accordance with the steam conditions thereof, and high and intermediate pressure rotors are fabricated from Cr-Mo-V steel and a low pressure rotor is fabricated from 3.5 Ni-Cr-Mo-V steel.
Turbines having a small capacity less than 100,000 and an intermediate capacity of 100,000 to 300,000 KW have a rotor small in size and thus if a material having both the advantages of the above materials used in the high and low pressure rotors is available, the high and the low pressure portions thereof can be integrated (fabricated from the same material). This integration makes the turbine compact as a whole and the cost thereof is greatly reduced. An example of a material of the rotor integrating high and low pressure portions is disclosed in Japanese Patent Publication No. 58-11504 and in Japanese Patent Laid-Open Publication Nos. 54-40225 and 60-224766.
If the high and low pressure portions are integrated by using the currently available rotor materials, i.e., Cr-Mo-V steel or Ni-Cr-Mo-V steel, the former cannot provide safety against the brittle fracture of the low pressure portion, because it is inferior in low temperature toughness, while the latter cannot provide safety against the creep fracture of the high pressure portion because it is inferior in high temperature strength.
The above-mentioned Japanese Patent Publication No. 58-11504 discloses a rotor integrating high and low pressure portions fabricated from a material consisting, by weight, of 0.15 to 0.3% C, not more than 0.1% Si, not more than 1.0% Mn, 0.5 to 1.5% Cr, 0.5 to 1.5% Ni, not more than 1.5% but more than 0.5% Mo, 0.15 to 0.30% V, 0.01 to 0.1% Nb, and the balance Fe, but it does not exhibit sufficient toughness after heated at a high temperature for a long time and thus long blades having a length not less than 30 inches cannot be planted thereon.
Japanese Patent Laid-open Publication No. 60-224766 discloses a steam turbine rotor fabricated from a material consisting, by weight, of 0.10 to 0.35% C, not more than 0.10% Si, not more than 1.0% Mn, 1.5 to 2.5% Ni, 1.5 to 3.0% Cr, 0.3 to 1.5% Mo, 0.05 to 0.25% V, and the balance Fe, and further discloses that this material may contain 0.01 to 0.1% Nb, and 0.02 to 0.1% N. This rotor, however, is inferior in creep rupture strength.
Japanese Patent Laid-open Publication No. 62-189301 discloses a steam turbine integrating high and low pressure portions, which, however, uses a rotor shaft fabricated by mechanically combining a material superior in high temperature strength but inferior in toughness and a material superior in toughness but inferior in high temperature strength, and thus it is not fabricated from a material having the same component. This mechanical combination requires a large structure to obtain strength and thus the rotor shaft cannot be made small in size and, in addition, the reliability is impaired.
Japanese Patent Laid-open Publication No. 63-157839 discloses a low alloy steel containing alloy composition for a steam turbine rotor, the Fe-base containing, by weight, 0.01-0.35% C, 0.35% or less Si, 1% or less Mn, 1.1-2.5% Ni, 1.5-3.5% Cr, 0.3-1.5% Mo, and 0.1-2.0% W. The rotor may contain at least one of 0.01-0.15% Nb, 0.01-0.10% N, and 0.002-0.015% B. However, the cited publication does not disclose a steel containing not more than 0.20% Mn and having the particular Mn/Ni ratio limited in the present invention. In addition, in the cited publication, there is no teaching of the important points of the present invention described hereinafter, i.e., that the steam inlet temperature of the steam turbine is made to be not less than 530.degree. C. and that the steam outlet temperature at the final stage blades is made not more than 100.degree. C.